But it is also a mater of computing time.
Making the missiles plain faster would be a lot cheaper and simpler processing wise then adding retargetting.
Yeah, that's the downside.
Like if they're kept slow, at longer distances, bigger fleet sizes, you'll have incredible amounts of missiles at any given moment, and we all know how bad the game can chug already in late game to begin with. Now imagine how bad it could get with hundreds of missiles recalculating targets every moment.
On the other hand just increasing their travel speed massively would actually end up in slightly better performance because there's a bit less stuff for the game to handle per second.
Only headache would be the need to rebalance the missile/PD stats around having missiles travel so fast.
The reason why I feel this is crude, though, is because it just would kind of feel weird and tacky if they had super fast travel times, like it would look kind of weird watching the missiles go ridiculously fast to their targets. The reason why they currently move so slowly is because it's a representation of the massive distances they're using sub FTL thrusters to cross, and it also is nice to watch a swarm of missiles make their way to a target to obliterate them (read: overkill), gives them a weighty, meaty feel.
But either way, something that feels and looks nice but doesn't actually work well is what most people would coin "useless", so I think just massively ramping missile speeds if retargeting proves to be an unnecessary addition to endgame lag would be as good solution as any.